ECOLOGY 149 
the one in salt-marshes the other upon _ sand-dunes. 
Many species have a circumpolar distribution* and often 
extend southward along the mountain ranges. Poa 
alpina, found at sea-level within the arctic circle, extends 
southward in the Rocky Mountains to Colorado where it 
is found on alpine summits. 
190. Circumpolar distribution Those species that 
are indigenous to North America and Eurasia usually 
show evidence, by a present circumpolar distribution, 
such as that of Poa alpina and many others, of a common 
origin in polar regions; or they suggest the probability 
of such distribution in the past. During a preceding 
warm epoch, when vegetation zones lay farther north 
than now, many species were circumpolar that afterward 
were driven south by the succeeding ice period. These 
species survived only where they found conditions suited 
to their requirements. Some were driven along the moun- 
tain ranges; others were driven along the coastal regions. 
As the climates of the northeastern coasts of North 
America and Asia are similar, there are many cases where 
the same or similar species of plants inhabit both 
regions.| Among grasses one notes the genera Diarrhena 
and Zizania, each represented by similar species in the 
two regions and not found elsewhere. 
191. Generic distribution—Sometimes large genera 
show a special development in certain areas although 
there may be scattering species in regions remote from 
the areas of greatest development. The genera Bouteloua 
and Muhlenbergia, mentioned above, are represented by 
numerous species on the tableland of Mexico, although 
certain species of the former are found as far south as 
*Hooker, ‘‘Distribution of Arctic Plants.” 
7Gray, “Analogy between the Flora of Japan and that of the United States.” 
